UIChart.vue Warning re: className

### Description

As I've been exploring further I enabled more detailed vue syntax logging. Long story short, the ui-chart element throws a warning message that doesn't impact functionality just puts stuff into the logs.

Message appears as:
[Vue warn]: Property "className" was accessed during render but is not defined on instance.

After researching the issue is the canvas tag has a bind to class from className, however className has no function for being defined and passed in the script portion either than the generic props.

The bind could be removed, set with a default value (if it is here for dynamic properties purposes only), changed to something static such as nrdb-ui-chart but since you gather class in the UI, I'm assuming the intention was to pass it along similar to the code in the UISwitch.vue template code: